[NEW] Handle system/private messages stream

[rafaelks]

@RocketChat/ios

Closes #1509

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@ca67cb7). Click here to learn what that means.
The diff coverage is 61.22%.

@@            Coverage Diff             @@
##             develop    #2180   +/-   ##
==========================================
  Coverage           ?   34.82%           
==========================================
  Files              ?      393           
  Lines              ?    18559           
  Branches           ?        0           
==========================================
  Hits               ?     6463           
  Misses             ?    12096           
  Partials           ?        0

Impacted Files	Coverage Δ
Rocket.Chat/Models/Message/Message.swift	100% <ø> (ø)
Rocket.Chat/Managers/Socket/SocketManager.swift	71.6% <0%> (ø)
...Managers/Model/MessageManager/MessageManager.swift	28.08% <0%> (ø)
.../MessageManager/MessageManagerSystemMessages.swift	100% <100%> (ø)
...Managers/Model/AuthManager/AuthManagerSocket.swift	26.38% <100%> (ø)

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